The manufacture of semiconductor devices involves numerous steps that are generally directed toward forming one or more layers on a semiconductor wafer substrate. For example, in the case of silicon wafers, a wafer is generally oxidized to form a layer of silicon dioxide on the surface of the wafer. All, or selective portions, of this silicon dioxide layer are subsequently removed via etching to expose the silicon wafer therebelow. Following etching, additional layers may be added to the etched silicon wafer, via thin film deposition or growth processes. These layers may then also be etched or otherwise treated to produce functional features on the surface of the silicon wafer.
The processes utilized to deposit and/or etch these layers, as well as the processes used to clean the wafers between steps, rely heavily on chemical reagents. In many instances, these chemical reagents comprise one or more gases dissolved in a liquid. Inasmuch as process parameters, such as the etch rate, quality of the etch, uniformity of the etch, and the like, are at least partially dependent upon on the concentration of the dissolved gas in the liquid, the concentration of the dissolved gas in the liquid is desirably controlled within tight specifications. If the concentration of the dissolved gas in the liquid is allowed to vary, undesirable manufacturing variations may result, or in the least, these chemical reagents will not perform optimally.
Unfortunately, controlling the concentration of a dissolved gas in a liquid to the degree necessary for such admixtures to be useful in many semiconductor manufacturing processes is difficult. As a result, variations in concentration can affect processing quality. For example, in some manufacturing processes of semiconductor devices, it is desirable to controllably etch copper so as to provide functional features on the surface of a semiconductor wafer. However, some copper etching applications may require etching precisely 5 nm of copper with less than a 5% variation. At least in this instance, variations in gas concentration are unacceptable.
Furthermore, although several methods of controlling the concentration of dissolved gas in a liquid are known, each of these prior art methods has limitations that render them inadequate for certain applications. For example, bubbling the gas that is to be dissolved directly into the liquid has been used as a method to dissolve gases into a variety of different kinds of liquids. Such a technique, however, does not optimize the quantity of gas dissolved or the amount of gas that remains in solution at the point of use. Control over the dissolved gas concentration when utilizing this approach is complicated by the pressure of the liquid into which the gas will be dissolved.
Additionally, several methods utilize cooling to increase the quantity of gas that may be dissolved into a liquid. Although such methods claim to increase the quantity of dissolved gas, these methods do not focus upon precise control of the concentration of the dissolved gas either at the outset or at the point of use. Thus, it is possible that the increased amount of gas, if any, that is dissolved into the liquid as a result of cooling the liquid will effervesce out of the liquid admixture at the point of use.
Thus, there is a need for an efficient method of establishing and maintaining a precise quantity of dissolved gas in a liquid admixture, not only to minimize the amount of gas that must be utilized, but also to provide liquid admixtures comprising sufficiently precise concentrations of dissolved gas so as to be useful for applications requiring tight specifications.